The present invention relates to a colour display means wherein use is made of the so-called guest-host interaction in a mixture of pleochroic dye material and cholesteric or chiral, nematic liquid crystals. More particularly the invention relates to a liquid crystal display means wherein electronic colour switching time is reduced and improved contrast of colour display is achieved.
Pleochroic dye material is dye material which absorbs light of specific wavelengths and displays a characteristic colour when the molecules thereof are aligned parallel to the electric vector of incident polarized light, but which transmits light almost unchanged, whereby the characteristic colour of the dye material disappears, when the long axes of the molecules thereof are perpendicular to the electric vector of incident polarized light. As reported by G. H. Heilmeier and L. A. Zanoni in Appl. Phys. Lett. 13 91 (1968), in some mixtures of pleochroic dye material, constituting the `guest` material, in nematic liquid crystals, constituting the `host` material, the molecules of the guest dye material tend to become aligned with the host liquid crystals and application of an electric field can change the orientation of the crystals and hence of the dye molecules. In other words, by successively applying and removing an electric field it is possible to produce changes in the optical density of the mixture and render the mixture alternatively coloured or colourless. Nematic liquid crystals employed may be of the so-called positive type, referred to below as Np material, in which normally, i.e., in the absence of a separately applied electric field, molecules are aligned with the long axes thereof parallel to the electric vector of incident polarized light, or of the so-called negative type, referred to below as Nn material, in which molecules are normally aligned with the long axes thereof perpendicular to the electric vector of incident polarized light.
A mixture of a pleochroic dye and nematic liquid crystals may be employed for colour display purposes in a cell such as shown in FIG. 1, which illustrates the case when Np material is employed, or in FIG. 2, which illustrates the case when Nn material is employed. In FIGS. 1 and 2, a mixture of nematic liquid crystal molecules 9 and pleochroic dye molecules 10 constitutes the dielectric in a cell defined by electrodes 3 and 4 which are shaped to define a letter or figure of a required display, or a segment thereof, and which are respectively applied on and supported by support plates 1 and 2. In the example shown, both support plates 1 and 2 and both electrodes 3 and 4 are transparent, and light may be transmitted through the cell, incident light, which impinges on the plate 2 side of the cell, being indicated schematically by the arrow 7 in the drawings, and transmitted light, which emerges from the opposite side of the cell, by the arrow 8. Needless to say, one plate and electrode supported thereby may be opaque, in which case only one electrode need define a required shape, or the required display segment may be defined by an open portion of an opaque support plate on which a transparent electrode is applied. An electric field may be applied on the host molecules 9 and guest molecules 10 which lie between the electrodes 3 and 4 by an AC voltage source 5, which connects directly to the electrode 4 and through a normally open switch 6 to the electrode 3.
In FIG. 1(a), since the nematic liquid crystals employed are Np material, when the switch 6 is unactuated and open, i.e., in the absence of an electric field applied by the voltage source 5, the host molecules 9 and guest molecules 10 are aligned with the long axes thereof generally parallel to the electric vector of incident light 7. Optical density of the cell is therefore comparatively high, as indicated by the solid curve showing transmittancy in FIG. 1(c), and transmitted light 8 is the characteristic colour of the guest dye material. When the switch 6 is closed, as shown in FIG. 1(b), the host molecules 9 and guest molecules 10 between the electrodes 3 and 4 are rotated through approximately 90.degree. about the central groups thereof and so become aligned with the long axes thereof generally perpendicular to the electric vector of incident light 7, with the result that there is a marked increase in transmittancy of the cell, as indicated by the dashed transmittancy curve of FIG. 1(c), and transmitted light 8 is more or less colourless. In FIG. 2, action is analogous when the host nematic liquid crystals employed are Nm material, but the cell is colourless when switch 6 is unactuated and coloured when switch 6 is closed.
Guest-host mixtures have definite possibilities with respect to use thereof in display devices, since the abovedescribed effects are achievable with an electrode spacing of approximately 5 .mu.m to 15 .mu.m and an applied electric field of the order of 1 V to 5 V. However, although such display means have good characteristics with respect to dynamic scattering and twisted nematic field effect mode (what is called TNFEM), conventional means have a major defect with respect to switching time, i.e., the minimum time required for effecting the cycle coloured-colourless-coloured in a cell employing Np material or the cycle colourless-coloured-colourless in a cell employing Nn material. It has been found that for Nn material a certain, although still unsatisfactory degree of improvement of switching time can be achieved by increasing applied voltage, there is an upper limit beyond which increase of applied voltage is of no further effect, in addition to which there is of course the disadvantage that power consumption is increased. It is thought that in a cell employing Nn material this phenomenum is due to the fact that electrodes of the cell are comparatively effective in aligning molecules of the guest-host mixture constituting the dielectric, and that for low values of applied voltage therefore there is slower, and also less complete, orientation of molecules of the guest-host mixture with the long axes thereof parallel to the electric vector of incident light. For both Np and Nn material it is possible to improve switching time by employment of a three-terminal drive system for causing forcible return of molecules to an initial orientation. However, this again has the disadvantage of increased power consumption, and also is a practical display means employing a plurality of cells such as shown in FIGS. 1 and 2 use of a three-terminal drive system for each individual cell results in a complicated control circuit for the display means as a whole, with consequent problems of installation and maintenance, as well as increased cost.
Another disadvantage of guest-host mixtures is that, although the mixtures afford contrast which is very little dependent on the angle from which a display is viewed, the absolute value of contrast achieved is low compared with that achievable in other display means.